Light sources of this basic type are ordinarily operated in the prior art as lasers in which an optical medium excitable to stimulated light emission is population inverted by external excitation, for example, by optical pumping, in order to generate stimulated emission on the population-inverted transition and therefore laser activity by resonant feedback in the resonator.
Optical media comprising a multilevel system, especially a three- or four-level system, are ordinarily used for such laser applications, in which laser activity occurs between two levels, the lower level of which is quickly depopulated in order to achieve the population inversion by excitation of the upper level.
A level in the multilevel system of the optical medium is often excited by electrical, chemical or optical excitation, in which case this excited state then breaks down internally to the upper laser level and a radiation transition to the lower laser level occurs that is then quickly depopulated to the base state.
Energy conversion therefore also occurs in such a laser process, especially wavelength conversion of the introduced shorter wave pump wavelength to the emitted longer wave laser wavelength occurs during optical pumping. The efficiencies of such laser light sources with optical media in optical resonators are sometimes very low and can therefore only be used with restrictions for energy or wavelength conversion but, on the other hand, have the advantage of high coherence and high monochromaticity.
The need for inversion of the medium also restricts the spectral working range, since appropriate three- or four-level laser media are not available for all optical wavelength ranges. For example, the increase in spontaneous emission with optical frequency hampers inversion of atomic transitions of very high transition frequency so that laser sources are only available to a very limited degree in the UV or x-ray range.
In lighting applications such high requirements are often not imposed on monochromaticity and coherence so that light sources other than the laser systems with restricted efficiency and wavelength range, in principle, could also be used, but unfortunately are not available in the prior art.